Writer/director Martin McDonagh’s latest film, “Three Billboards Outside Ebbing, Missouri,” is an angry, hot-blooded tale, seething with rage and energy. It’s a barn burner, a bracing shot of whiskey downed while spoiling for a fight, a cathartic wail against the zeitgeist of rape culture and state brutality. It’s a rallying cry, a right hook to the jaw, and wow, does it ever hurt so good.

Frances McDormand swaggers onscreen to Carter Burwell’s guitar-strummed score like a lone gunfighter in the Old West. Her Mildred Hayes isn’t slinging bullets, though, just words, but they pierce just the same. Her words, plastered onto three blazing billboards she rents on a deserted country road, are a plaintive wail of grief, a cry for help. Her daughter, raped and murdered, has been dead for months. No arrests have been made. So Mildred turns to advertising to demand some answers from Chief Willoughby (Woody Harrelson).

McDonagh’s sweet spot is brutal, dark, foul-mouthed comedy from the mouths of flawed but brilliant characters that you actually care for, as seen in the brilliant “In Bruges,” and “Seven Psychopaths.” “Three Billboards” masterfully achieves that with an even higher degree of difficulty, tackling sexual assault and police brutality with equal parts satire and humanity.

It’s thanks to the generous writing and fearless performances that it looks so easy. McDormand is absolutely riveting, in what will be one of the defining roles of her career, but Sam Rockwell has a much harder task as Jason, a police lackey of Chief Willoughby’s who isn’t the sharpest tool in the shed. His character is dumb, violent, impulsive and a laughingstock, but somehow, against all odds, we end up rooting for him. These characters aren’t all hero or all villain, but somewhere in between, and the tragic, comic Jason beautifully expresses those extremes.

Mildred is a hero who seems to have crawled from the depths of our injured souls. Flawed, human and trying her best, she’s mad as hell and not taking it anymore. When justice fails, Mildred, who sells ceramic bunnies at the local gift shop, tosses a Molotov cocktail at justice’s front door.

The militant Mildred who comes to the surface captures an elemental collective female anger that’s been bubbling for years, boiling over since last fall. We’re sad, mad and sick of it, and Mildred is the mythical creature who holds our anger, makes it manifest, hurling off invective and foul-mouthed insults like sonnets. But she’s vulnerable too. While “Three Billboards” is an invigorating fable of righteous vengeance, it espouses a trenchant message about the purifying power of forgiveness — and it’s hilarious, to boot.